When operating an internal combustion engine in the HCCI mode (homogenous charge compression ignition) sometimes also called CAI (controlled auto ignition), ATAC (active thermo atmosphere combustion) or TS (Toyota Soken), the air/fuel mixture is not ignited by a spark ignition, but by controlled self ignition. The HCCI combustion process can, for example, be evoked by a large amount of hot residual gases and/or by a high compression and/or a high inlet air temperature. One requirement for self ignition is a sufficiently high energy level in the cylinder. Internal combustion engines that can be operated in the HCCI mode, regarding both spark ignition and diesel engines are known, see e.g. U.S. Pat. No. 6,260,520, U.S. Pat. No. 6,390,054, DE 199 27 479 and WO 98/10179.
Compared with a conventional spark-ignited combustion, the HCCI combustion has the advantage of reduced fuel consumption and limited harmful substance emissions. However, it is not easy to regulate the combustion process and particularly to control the self ignition of the mixture. Therefore, a regulation of controlling variables for e.g. the fuel injection (injection volume or injection point and duration), internal or external exhaust gas re-circulation, inlet and outlet valves (variable valve control), exhaust gas counter pressure (exhaust flap) is required. If necessary an ignition support, air inlet temperature, fuel quality and compression ratio (for internal combustion engines with changing compression ratio) can be regulated. However, in order to be able to regulate these process-influencing variables in the sense of an optimum combustion process perhaps with regard to efficiency, engine protection, quiet running, noise development, harmful substance emissions and fuel consumption, instruments for evaluating the quality of the combustion process are required.
In the case of previous solutions one was generally only restricted to evaluating individual measurable output variables of the combustion process. A particularly expressive variable is the combustion chamber pressure. This can, e.g., be detected with indicator crystals that are directly introduced into the cylinder head or with piezopressure sensors that are integrated in the cylinder head gasket. Another possibility is an ion emission measurement by which the combustion process can be evaluated qualitatively. Apart from these solutions being complex and costly, they only conditionally allow a comprehensive and precise evaluation of the quality of the combustion process.